4 week trip through Asia (predominantly) solo

So some background for your benefit/context. 27/m from Perth, Australia. Just had a short trip through Singapore and Malaysia and had such a great time I want to do some more exploring of Asia later this year, my plan is to do this in September/Early October (for reasons Ill explain below). Ive never done a trip of this length before so it will really be my first overseas trip solo, I have my heart set on most of the locations but wanted to seek advice on whether the time spent there is enough to get my moneys worth or not.

Here is my tentative plan in order of locales (Stop 1. and 9. are set in stone, but willing to be flexible with the rest) –

Singapore – 3 days (have a close friend who lives there who I want to see)
Bangkok – 2 days
Cambodia – 2 days
Ho Chi Minh – 2 days
Saigon – 3 days
Hanoi – 2 days
Hong Kong – 3 days
Manila – 3 days Bali, Indonesia – 5 days (will be meeting back up with my family there as its my mothers birthday, so purely a rest, relaxation, celebration before heading back to Perth).

So my questions are firstly is the length of time in each country sufficient in your view given a lot of them are only a few days? (Im interested mostly in the countries history and visiting some famous landmarks, and of course sampling all the great food and drink).

Secondly Im very keen to visit Japan, though Im conscience a few days there wouldnt be enough, would it be realistic to fit it in somehow (I think at most I could extend the trip by one more week)? Or should I look to plan a seperate trip just to visit there for a few weeks?